1. Field of the Invention
The present invention relates to a composition in particulate form comprising a platinum-containing hydrosilylation reaction catalyst and an organosilicon material. More Particularly, the present invention relates to a particulate form of a platinum-containing hydrosilylation catalyst that will not cause premature curing when incorporated into organopolysiloxane compositions which cure by a hydrosilylation reaction.
2. Description of the Prior Art
Due to their unique advantages of a rapid curing rate and the absence of by-product evolution during curing, organopolysiloxane compositions which cure by a hydrosilylation reaction are widely used as silicone rubber compositions, coating compositions, and the like. Such organopolysiloxane compositions typically include at least one diorganopolysiloxane with at least two alkenyl groups in each molecule, an organohydrogenpolysiloxane with at least two silicon-bonded hydrogen atoms in each molecule, in an amount sufficient to bring about the curing of the diorganopolysiloxane; and a platinum-type catalyst. However, such organopolysiloxane compositions suffer from an inferior storage stability.
U.S. Pat. No. 4,293,677, which issued to Imai on Oct. 6, 1981 describes encapsulating organohydrogensiloxanes using complex coacervation and in-situ polymerization, two of the most common microencapsulation techniques. In accordance with example 1 of this patent, an aqueous gelatin solution adjusted to a pH of 9.5 was added to a solution of a trimethylsiloxy terminated polymethylhydrogensiloxane in molten paraffin wax. A 10 percent by weight solution of gum arabic is then added and the pH of the resultant emulsion is adjusted to 4 over a period of two hours to precipitate the mixture of the two polyelectrolytes that forms the encapsulant. The encapsulant is cured by gradually heating the dispersion of coated particles to a temperature of 50 degrees C.
The in-situ polymerization process exemplified in example 2 of the Imai patent involves polymerization of styrene in the presence of a dimethylsiloxane/methylhydrogensiloxane copolymer as the dispersed phase of an emulsion wherein the aqueous phase contains a solubilized polyvinyl alcohol and potassium persulfate.
A disadvantage of encapsulating the organohydrogensiloxane reactant as taught by Imai et al. is the relatively large amount of encapsulating polymer that is introduced into the composition. Many of the thermoplastic organic polymers suitable for use as encapsulants are incompatible with the reactants present in the curable composition. The presence of relatively large amounts of incompatible polymers may detract from the appearance, physical properties and optical properties of the cured material.
One way to reduce the amount of incompatible encapsulating polymer introduced into a curable composition is to encapsulate the platinum-containing catalyst rather than the organohydrogensiloxane reactant as taught by Imai et al. One of the most effective classes of platinum-containing catalysts for curing organosiloxane compositions of the type described in the Imai et al. patent are reaction products of an inorganic platinum compound such as hexachloroplatinic acid with liquid vinyl-containing organosilicon compounds such as sym-tetramethyldivinyl disiloxane. The solution can then be diluted to the desired platinum content, typically between 0.1 and 1 percent by weight, using a liquid dimethylvinylsiloxy terminated polydimethylsiloxane. Alternatively, the undiluted reaction product can be used as a catalyst.
The prior art has attempted to improve the storage stability of platinum-containing hydrosilylation catalysts. One method for accomplishing this is disclosed in Japanese Patent Application Laid Open [Kokai] Number 49-134786 [134,786/74], which teaches the preparation of a platinum-type catalyst in powder form. Silicone resin and a platinum compound catalyst adsorbed on a finely divided material such as quartz powder, are mixed and kneaded on a hot roll and the resulting mixture is pulverized or ground.
U.S. Pat. No. 4,481,341, which issued to Schlak et al. on Nov. 6, 1984 describes thermosetting organosiloxane compositions comprising a polyorganosiloxane containing at least two ethylenically unsaturated hydrocarbon radicals per molecule, a polyorganohydrogensiloxane containing at least two silicon bonded hydrogen atoms per molecule and a platinum-containing catalyst that is dispersed in a finely divided, solid material, such as a silicone resin, at a concentration of from 0.001 to 5 percent by weight of platinum metal.
The finely divided material in which the catalyst is dispersed is virtually insoluble in either the aforementioned polyorganosiloxane or polyorganohydrogensiloxane and melts or softens at a temperature between 70 and 250 degrees C. The alleged advantage of these compositions disclosed by Schlak et al. is that the catalyst remains isolated from the other ingredients of the curable composition until the composition is heated sufficiently to melt the material in which the catalyst is dispersed. Because the organosilicon compounds present in the composition will not cure in the absence of the catalyst the composition can allegedly be stored for long periods of time without undergoing curing or even an increase in viscosity.
A disadvantage of Schlak et al. catalyst resides in the method used to prepare the catalyst composition. A solid block or sheet of resin containing the platinum composition dispersed throughout is ground to a fine powder. The random nature of the grinding operation makes it possible that some of the particles will contain platinum catalyst on their surface. Even trace amount of platinum have been shown to cause premature curing of the type of organosiloxane composition exemplified in this patent.
U.S. Pat. No. 4,784,879, which issued to Lee et al. on Nov. 15, 1988 describes the preparation of an encapsulated form of a platinum-containing hydrosilylation catalyst. The platinum-containing catalyst is encapsulated within one or two layers of thermoplastic organic polymers. The catalyst is prepared by polymerization or precipitation of the encapsulating polymer in the presence of the catalyst. The resultant microcapsules are then washed with a liquid that is a solvent for the catalyst but not for the encapsulating polymer. The exemplified compositions require one washing with methanol and one with cyclic dimethylsiloxane oligomers to ensure their storage stability when incorporated into a curable organosiloxane composition.